1. Field of the Invention
The field of this invention relates to purifying graphite, especially natural graphite.
2. Prior Art
Graphite is a form of elemental carbon crystallized predominantly in the hexagonal system. Silicate mineral impurities varying in kind and percentage are usually associated with graphite in the ore.
The uses of natural graphite are dependent upon its physical and chemical properties. It is unctuous, which accounts for its demand as a dry lubricant and which, combined with its high electrical conductivity, makes it useful for motor and generator brushes. It is soluble in molten iron and, therefore, is used to raise the carbon content of steel. Its largest single use probably still is for foundry facings, where it prevents metals and alloys from sticking to the molds. Graphite is unequaled for many refractory uses, including crucibles, because of its high heat conductivity, its slow burning property and its ability to retain good strength at high temperatures. It is probably best known, however, for its use in such products as pencils, batteries, paints and inks, and brake linings.
Natural graphite is sold in several different grades. Low grade (low purity) graphite has several substantial uses.
For example, foundry facings have accounted for nearly 20 percent of the total consumption in the past years. Low quality and amorphous graphite is suitable for this use. The graphite is mixed with a small amount of clay, suspended in an adhesive material and applied as thin coatings to mold surfaces to provide for clean and easy recovery of the metal castings.
Another large user of low quality graphite is the steel industry. The graphite is added to steel melts to increase the carbon content to the desired level.
Other uses of graphite require that the graphite have very high purity. High purity natural graphite is required, for example, when the graphite is employed in lubricants or in forming high temperature refractory crucibles. These high purity (premium grade) natural graphites are substantially more valuable than low grade graphite. Flake graphite from disseminated deposits must be concentrated to meet market requirements. Virtually every known concentrating device and combination of separating principles has been tried in an effort to upgrade or purify natural graphite. The mineral has gained a reputation of being difficult to concentrate and upgrade. It has been reported that a high proportion of mills built to refine or purify natural graphite have failed to make commercial recoveries. Graphite actually is one of the easiest minerals to segregate into a rough concentrate, but one of the most difficult to refine into a high purity product.
Heretofore, attempts to purify graphite have involved various conventional flotation techniques. Although high recoveries are common employing prior art flotation techniques, concentrates with acceptable graphitic carbon content are difficult to attain and, indeed, with some ores, impossible. The chief problem lies with the depression of the gangue minerals. Relatively pure grains of quartz, mica and other gangue minerals inadvertently become smeared with fine graphite, making them floatable such that it becomes next to impossible to obtain a high grade natural product by flotation.
Seemingly, no mechanical process will yield flake graphite of extremely high purity. High purity products can be obtained by employing certain costly chemical processes. For example, silica mineral matter can be eliminated from graphite with HF or by fusion with an alkali such as sodium carbonate. These chemical processes are less than satisfactory, however, because they are generally costly, employ hazardous chemicals and/or extreme operating conditions.
Clearly, there remains a present need for a simple process to obtain a high purity graphite material.